The present invention relates to electrophotographic carrier particles, and particularly to carrier particles for electrophotographic developers comprising a fluoropolymer coating, wherein a monomer species is grafted to the fluoropolymer.
Electrophotographic processes and apparatus employ the use of developers to form visible images that are typically transferred to and fixed on a paper sheet. The developers comprise a toner, which typically comprises a resin and a colorant, along with other desirable additives such as charge control agents. In general, a desired image is formed on an organic photoconductor (OPC) coated medium such as a drum or belt in the form of a charged pattern representing the image. Toner is electrically attracted to the charge on the drum and adheres to the drum in an imagewise manner. Then, the toner image is transferred from the OPC medium to an image-receiving substrate (typically paper) and fused, resulting in permanent image formation.
In many development systems, charge is imparted to the toner triboelectrically by mixing toner particles with carrier particles, typically, particles about 20 to 200pm in diameter. In magnetic brush development systems, the carrier particles are preferable resin-coated ferromagnetic particles. The toner particles adhere to the oppositely-charged carrier particles and are conveyed to the photoconductor where the toner is attracted to and deposited on the oppositely-charged latent image areas of the photoconductor. The carrier particles are collected and recycled for remixing with additional toner.
Because the carrier is a recyclable component of the developer, it is desirable to make the carrier last as long as possible, to minimize cost of usage. After a period of use, toner particles tend to irreversibly adhere to the carrier, rendering triboelectric charging ineffective and necessitating replacement of the carrier. This is a problem sometimes referred to as "toner filming" or "scum" and can be found when using fluoropolymer coating materials such as polytetrafluoroethylene (PTFE). However, such fluoropolymer materials are triboelectrically desirable for use in making the carrier. Typically, fluoropolymers have a low surface free energy due to the presence of carbon-to-fluorine bonds and, as such, make ideal materials for carrier coating. Toner filming or scum may be suppressed by incorporating certain silicones and copolymers of tetrafluoroethylene (TFE), p-vinylidene fluoride, and the like. The lack of adhesion problem has been addressed by the provision of another agent such as a heat-curable epoxy system to adhere the PTFE to the substrate, but this solution is less than desirable because the presence of the epoxy alters the characteristics of the end-product carrier material.
Another problem with prior art developers relates to solid area development and the control thereof. In carrier/toner systems, uniform application of toner across a relatively large image on the document is desired. This is commonly called "solid area fill." During formation of a latent image on the photoconductor surface, an electric field is formed of the size and shape of the optically projected image (i.e., "imagewise"). Electrostatic field lines of force tend to migrate to the edges of the latent image field and toner, during development, is deposited along these lines of force. If the shape of the field is not corrected, most toner will be deposited along the edges of the latent image field, resulting in little or no development of the interior of the image, a condition known as "hollow character defect" or "edging."
One solution to correct this defect is to move a conductive bar or the like into the field, whose force lines project into space. This has the effect of making the field lines project perpendicularly to the photoconductor surface and to space themselves evenly across the large solid image field. This effect is commonly known as the "development electrode effect."
Ferromagnetic carriers used in magnetic brush development take the place of solid development electrodes. If they are sufficiently conductive, the carrier particles render excellent solid area fill to large image areas. The conductivity of the carrier particle determines the strength of the development electrode effect.
Examples of carrier core materials used in the prior art range from extremely resistive flint glass (which is only able to develop solid areas not larger than ordinary type fonts) to powdered iron and steel, which develops excellent solid area fill. However, particles containing iron can be highly susceptible to rusting in high moisture environments, or the formation of "scale," which interferes with carrier coating adhesion. These core materials generally must be passivated and cleaned, either chemically or by surface oxidation.
Synthetic ferrite core materials are not rendered useless by moisture, because they are formed from metal oxides. They are more resistive than iron and more conductive than glass beads. To improve their solid area image development, however, it is usually necessary to incorporate electroconductive particles in the coating to enhance the development electrode effect.
Another problem encountered with carriers having fluoropolymer coatings is that such coatings can impart excessive triboelectric charge to positive (+) toners, resulting (i) in decreased toner development and lower image density than desired or (ii) in excessive attraction of toner to carrier, resulting in high toner concentration leading to "background" on developed copies.
Accordingly, improved development systems including improved carrier particles continue to be desired.